A known heat-source system used to supply chilled water and provide local heating and cooling in semiconductor factories includes a plurality of centrifugal-chillers that can be activated or deactivated according to the quantity of heat required by an external load. The heat-source system includes, besides the centrifugal-chillers, cooling-water pumps for supplying cooling water to condensers of the centrifugal-chillers, cooling towers for cooling the cooling water heated by recovering heat of condensation in the condensers by bringing the cooling water into contact with the outside air, and chilled-water pumps for supplying the chilled water cooled by evaporators of the centrifugal-chillers to the external load. Furthermore, the cooling towers have cooling-tower fans for introducing the outside air into the cooling towers. PTL 1 (described below) discloses an invention related to such a heat-source system, for improving the operating efficiency of the overall heat-source system, taking into consideration not only the chillers alone, but also auxiliary machines, such as the cooling-water pumps, the cooling towers, and the chilled-water pumps. More specifically, a table is formed, with which the COP of the overall heat-source system can be understood from the relationship between the outside-air wet-bulb temperature and the chiller load factor. Next, parameters used in an arithmetic expression which maximizes the COP of the overall heat-source system is determined from the table. Then, based on the result of calculation, the number and output powers of chillers to be operated and the flow rate and temperature of the cooling water are controlled.